I have central air with ducts running through the hot, uninsulated attic (there is blown-in insulation between the joists to insulate the rooms below). Fortunately it's only about 25 feet of ductwork, but it's a super key length!
The ducts are the standard insulated ducts–about R-8–which seems far too little insulation for the temperature differential between the cold air inside and the hot air outside. So I want to add more insulation to the ducts.
They run across the top of the ceiling joists (perpendicular) which makes it slightly more tricky.
What I'm struggling with is how to balance the importance of an air-tight seal vs. a thick, circular seal–let me explain with two contrasting approaches.
Approach 1
Buy fiberglass insulation batts and wrap them like a spiral around the ducts. There will be a lot of gaps.
Approach 2
Use rigid foamboard to create a box around the ducts, and tape the corners airtight. This will be more airtight but will leave an air-gap between the duct and the insulation.
I suppose another approach would be to buy some pre-insulated ductwork that is large enough to completely envelop the smaller ducts, so I'd get a perfect air seal plus a circular insulation, but this would lead to a lot of potential confusion for future contractors, and would probably be needlessly expensive.
Best Answer
Adding additional insulation can help a little bit but even R30 around ductwork really won’t pay back.
The R8 blanket or if flex pipe is really only stopping condensation.
The small air space is difficult to fully insulate I have seen foam board used and even spray on foam and with the system in an unconditioned air space even massive insulation R60 the air in the ducts still warms up when the system is idle.
The short time of warm air is due the difference in temperature.
Will additional insulation help sure but on hot days when the system starts blowing there may be a few seconds of warmer air because of the small area of the duct.
Early on I tried to eliminate that small warm blast by completely sealing , spray foam and even 2 layers of R30 insulation.
Small air voids in your case won’t be a problem as long as there is no pathway for air to flow, however I found that the cost verses results to be disappointing and will add when the owner wants but mention my early experience trying to do the same. On several jobs where I was asked to add additional insulation the owner thanked me later for suggesting to power ventilate the attic space and found this to be a better solution because the temperature in the unconditioned airspace was reduced and the $ spent on this were more effective.